Oo! This game sounds fun. While I don't really have too many "Deal breakers", there are things I dislike in a system.
1. "Build Heavy". If the book has a hundred feats and eight classes, okay, I can deal with that. But if it has five hundred feats and sixteen classes, no thanks. I looked at GURPS for all of twenty seconds before I realized, with all of its modifiable traits, advantages, and disadvantages, that it is not for me. Why? Because I run games, and when systems like this are played, I wind up seeing two or three PCs that are amazing, one or two that are about par for the course, and maybe one or two that are completely useless.
And I have to spend the entire game trying to engage uber PCs while watching the weakling PCs fight at the edges of the encounter. And I hate having to game the system just to challenge my PCs. I realized 4e wasn't for me when I realized even a natural 20 wouldn't hit one of the party rogues - but the same monster would hit the other party rogue on a 10+. I hate that.
2. Fixed Progression. I don't really care one way or another whether a game is class-based. But I dislike games where almost all character classes mean nothing after character creation, or where you're "locked in" from character creation. I always hated seeing people make "20 level builds" in 3e, for example.
There needs to be a way for characters to gain new abilities that they couldn't predict they'd need at character creation. Though, I'm going to list an exception to my own rule - if the game is rules-light, or PCs have a limited range of inputs into their character, I will waive this rule, as "character growth" becomes hardwired into the game in other ways.
3. "Dark". I like dark games, but I dislike how "dark" is interpreted in most RPGs, because it always comes off as this sort of super gothy crap that makes me think I'm supposed to be sitting in a dark room listening to cutter music and complaining about the price of eyeliner. And a lot of the "dark" is just over the top and silly: "oh! This villain uses aborted fetuses that he charges with the necromantic energies of his mother's soul to create fetus grenades... which he uses to blow up orphanages so he can use the power to create an army of ressurected serial killers who..."
Paizo has a bit of this in their mechanics and flavour - the witch has a power that lets her eat children, for example, and there's a lot of supervillain stuff in the Alchemist class. Not a fan, personally.
4. Assumed character role. If the game assumes PCs are of one particular group, I get annoyed sometimes. Usually if that assumption is rather tight. Hey, you know what? I don't want to BE a vampire. Or an arthurian knight. Or a superspy.
5. Tight Classes, broad world. Games where the character classes are very "tight" in describing a role... and there are only a few of them. Gimme broad character classes that can describe many specific roles, or a few tightly-defined character classes that fit in a very tightly-defined world. But if I'm playing in a sci-fi universe, your game needs more than a pilot, marine, and sniper class.
On a related note, I'm not a fan of games that can only really tell one type of story.
6. Non random PC Gen. Not a deal breaker, but I kind of like having some aspects of your character be random. It opens up play a bit, and breaks people out of always playing the same character.
7. "Hero to Zero". I'm not a fan of PCs starting out as complete losers, and then progressing to awesome demi-god status. I prefer games with broader power curves, if only because this allows new PCs to be introduced with zero experience points.
8. Equipment Reliance. Not a fan of any game where the PCs' gear is a huge factor. Even in games where the setting makes PC gear choices important (Shadowrun, for example), I am just not a fan.
9. PC Mechanical Control. I do not like any game where the Players have the ability or expectation to always say something is true. I do not like games where PCs can have powers that are not able to be countered by a GM, and the Players have the expectation that they can circumvent "rule zero".
10. Specific Worlds, but Vague Play: Games that were obviously designed as settings first and foremost, and games second. I don't like worlds that basically have setting restrictions that get in the way of play, or worlds that read better than they play. And I am not a fan of games that set out how to create characters, how to create an adventure, how skills work, how combat works, how the setting works... and then doesn't tell you what the characters actually DO.
I remember playing Shadowrun 2e and having NO idea that "Shadowruns" were criminal in nature. And Mechwarrior was fun for about an hour, until we realized the setting was designed in such a way that we had no idea what our characters could do outside of the mech that was at all interesting. And don't even get me started on Warhammer Fantasy Roleplay...
11. Lovecraftian: I love HP Lovecraft. We even have the same birthday. But these days, Lovecraft has kind of become a sellword. And when I see something that "draws upon the works of H.P. Lovecraft for inspiration", I run for the hills. About the only RPG I want to see that is Lovecraftian is Call of Cthulu. And a few D&D monsters.
12. Hit Location Systems. Never found one I liked. Always been a pain in the butt. Ditto for called shots.
13. Critical Fumbles. Unless they're very simple, and minor in effect - but generally, they lead to deadpanery which is just a PITA.
14. Clunkiness. Using THAC0 instead of BAB is a very good example (attack tables make sense, but if you want to adapt THAC0 as your base, you may as well just go to the BAB route). Likewise, if you run a percentile system, and your base score is, say, 53%, and you get a -25% penalty, actually writing the game to expect the player to subtract 25 from 53 to figure out what he needs to roll (psst.... 28 or under), I hate your game. You should write it so that players know they need to roll BETWEEN 25 and 53 for success.
And yet so many games don't do this.
15. Big numbers. Hate games with big numbers. Buckets of dice? Fine, but only if I don't need to add the results. I hate seeing "1d20+43" and then seeing "8d8+23 damage". Seriously. 1d20+6, 2d8+1 damage is about as high as I want to go.
16. Interrupts. I am not a fan of the premise, because it leads to all the players and GMs talking over one another to trigger powers. And, again, a certain type of expectation in play.